Method of Sections Question (1 Viewer)

[perhapsmaybewhy]

This question has me completely stumped. If anyone could show me how to do it, it would be greatly appreciated.
(probably won't get a reply considering this sections essentially a ghost town.)

 

[Rathaen]

If you take moments around joint number 1, forces X and Z pass through the joint, and so the only force resisting the turning moment of the external forces is force Y (since the system is in equilibrium). You'll have to work out the perpendicular distance between Y and Joint 1 (some combinatione of Sine rule, Cosine rule and pythagoras, usually), and then take moments around joint 1 = 0, and taking clockwise (because why not) direction as positive.

That gives: (20kN * 2) + (20kN * 4) - (Y * distance) = 0. Which lets you work out Y.

Once you have Y, take another joint that X passes through, but not Z (or the other way around, doesn't matter too much), in other words, Joint 2 or 3, and take moments around that to find force Z. Then you can find out force X by doing the same thing: Take moments around any joint that X DOESN'T pass through to find X.

That's the explanation of HOW to do it. If you really can't do that, your teacher really should be helping you on this, man.

 

[BlackKlavier]

yea that method works just fine (Y)
and yes your teacher should be helping you with this. it's a foundation thing you need to know.
Here's what i believe is an easier method of thinking of method of sections:
- generally the toughest you'll get is three unknowns like in this question.
so eliminate 2 unknowns. like in rathaen's answer:
take moments around 1 to eliminate X and Z to find Y
then take moments around 4 to eliminate X and Y to find Z
then take moments around 7 to eliminate Y and Z to find X

so always think: i have three find a spot to take moments such that i eliminate two.
rathaen's method works perfectly as well only difference is that using 1 4 and 7 allows you to assess the members in whichever order you want. so if the question asks for only one (and you still have 3 unknowns) then you minimize the work you need to do by using this though structure.